Carbonate Beach Sand of Abaiang Atoll, Kiribati: Geochemistry, Biogenic Sources, and Properties

Abstract

Sandy beaches are critical resources for low-lying Pacific atoll communities, providing protection during storms, and land area for many coastal villages. Information on the nature of atoll beach sediment, its geochemistry and composition, can help to establish priorities to effectively protect the sources of Pacific island beach sediment. To understand sand sources, this study evaluated its physical characteristics including grain-size, geochemistry and composition, from windward and leeward beach profiles around Abaiang Atoll, Kiribati. Beach sand was >99% carbonate, averaging 37% coral fragments, 30% mollusc shells, 12% foraminifera, and 20% calcareous algae. Significant differences were found between reef and lagoonal sites in proportions of coral and mollusc fragments and foraminifera tests, with lagoon beaches having higher mollusc and coral proportions and lower foraminifera relative to reef beaches. This is attributed to high foraminiferal productivity offshore of reef beaches, and taphonomic durability of coral fragments in longshore drift into the lagoon. Mean sediment diameter increased from the upper to lower beaches at all sites, but fine sediment was lacking, attributed to its dissolution by rainfall and groundwater outflow. Geochemical analysis showed a mean of 84% Ca-Mg carbonates, of which 80% was calcium carbonate. There was no significant difference in the mean calcium percentage or calcium carbonate composition of the sediment between lagoon and reef beach sediment sources. Magnesium and magnesium carbonate content were significantly higher at reef sites relative to lagoon sites, attributed to higher proportions of foraminifera. Sediment-producing near shore habitats are critical to village protection through provision of beach sand, and this study shows the need to better conserve and manage coral reefs and habitats such as lagoon seagrass beds, to ensure continued atoll beach sand supply.

Juvenile corals underpin coral reef carbonate production after disturbance

“…The Intergovernmental Panel on Climate Change (IPCC) predicts a climate‐driven sea‐level rise of 0.43 m–0.84 m by 2100 (Oppenheimer et al, 2019), thus increasing the risk of coastal flooding, especially during tropical storms (Ellison et al, 2019; Nunn et al, 2017; Tebaldi et al, 2012). Sea‐level rise will be amplified in the tropics, where vulnerable ecosystems such as mangroves and coral reefs act as natural barriers to protect more than 500 million people from oceanic waves (Hoegh‐Guldberg et al, 2007).…”

Atoll inland and coastal mangrove climate change vulnerability assessment

“…Coral reefs and seagrasses are primary sites of sediment production on atolls such as Jaluit, and associates like parrotfish (Scaridae) enhance the sediment supply through bioerosion (Woodroffe 1992 ; Kench and Cowell 2002 ; Horstman et al 2014 ; Perry et al 2015 ). Atoll lagoon mangrove margins in Kiribati showed sediment derived from corals, molluscs, foraminifera and calcareous algae (Ellison et al 2019 ), indicating the importance of adjacent reefs and seagrasses to mangrove sediment supply. The sedimentation sources of Jaluit mangrove shorelines and interior sites both require future on-ground assessment.…”

“…Atolls form on coral platforms, therefore depend on coral reefs for geological formation (Grigg 1982 ). Reefs also buffer from persistent wave action and storm events, and provide habitat for foraminifera which also contribute sediment supply (McLeod and Salm 2006 ; Beger et al 2008 ; Ellison et al 2019 ). The threat of coral bleaching events with warmer ocean temperatures is expected to become more likely with climate change, a process which is difficult to mitigate (Sully et al 2019 ).…”

“…Where minerogenic sediment is low or absent, mangrove sediment accretion derives from biogenic sources such as root mat development (McKee et al 2011 ). Atolls, 436 in total globally (Goldberg 2016 ), have very low elevations and rely solely on sediments derived from coral reefs and associated molluscs, calcareous algae and foraminifera (Ellison et al 2019 ). This study investigates the mangroves of Jaluit Atoll located in the Republic of the Marshall Islands (RMI) in Micronesia.…”